Hot dip galvanized steel sheets are widely used in automobiles, electrical appliances, and other apparatuses owing to their good corrosion resistance compared with ordinary cold-rolled steel sheets. The hot dip galvanized steel sheets in these uses are often press-formed. The hot dip galvanized steel sheets have, however, a drawback of inferiority in press-formability compared with the cold-rolled steel sheets because the galvanizing components in the hot dip galvanized steel sheet adhere with the press die thus making the sliding resistance between the steel sheet and the die large and instable compared with that for the cold-rolled steel sheets. That is, for a hot dip galvanized steel sheet, the steel sheet becomes difficult in sliding into the die during the press-forming stage at a portion such as bead part where the sliding resistance increases, which likely induces fracture of the steel sheet.
A common practice to improve the press-formability of zinc-based plated steel sheet is a method of coating a high viscosity lubricant oil. The method, however, has problems such as the generation of defects during the painting stage caused by insufficient degreasing, and the instable press-formability during the press-forming stage caused by absence of the lubricant oil. To solve these problems, minimization of the quantity of lubricant oil is an effective means. To do this, however, the improvement in the press-formability of zinc-based plated steel sheet is required.
The galvannealed steel sheet is a hot dip galvanized steel sheet which formed an Fe—Zn alloy layer thereon after heating thereof. The alloy layer is normally composed of Γ phase, δ1 phase, and ζ phase. When the Fe concentration decreases, the alloy layer tends to decrease in hardness and melting point in an order of Γ phase→δ1 phase→ζ phase. From the viewpoint of sliding performance, the Γ phase with high Fe concentration is effective because of the high hardness, the high melting point, and the hardly-inducing adhesion. Accordingly, the galvannealed steel sheet which emphasizes the press-formability is manufactured so as to have a high average Fe concentration in the alloy layer.
When, however, the average Fe concentration in the alloy layer increases, the Γ phase which is hard and brittle is likely formed at the interface between the plating and the steel sheet, thereby likely inducing a phenomenon of peeling of plating (what is called the “powdering”) in the vicinity of the interface during the press-forming stage.
JP-A-1-319661, (the term “JP-A” referred to herein signifies the “Unexamined Japanese Patent Publication”), discloses a method of forming a hard iron-based alloy as the secondary layer on ordinary alloy layer using electroplating method or the like to attain both the sliding performance and the powdering resistance. The double plating layer, however, increases the manufacturing cost.
Further low cost methods are disclosed in JP-A-53-60332 and JP-A-2-190483. According to these disclosed technologies, the weldability and the press-formability are improved by forming an oxide film composed mainly of ZnO on the surface of a zinc-based plated steel sheet applying electrodeposition treatment, dipping treatment, coating oxidation treatment, or heat treatment.
JP-A-4-88196 discloses a technology to improve the press-formability and the phosphatability by forming an oxide film composed mainly of a P oxide on the surface of a zinc-based plated steel sheet by dipping the steel sheet in an aqueous solution of pH 2 to 6, containing 5 to 6 g/liter of sodium phosphate, by applying electrodeposition treatment in the aqueous solution, or by spraying the aqueous solution onto the steel sheet.
Furthermore, JP-A-3-191093 discloses a technology to improve the press-formability and the phosphatability by forming a Ni oxide film on the surface of a zinc-based plated steel sheet by applying electrodeposition treatment, dipping treatment, coating treatment, coating oxidation treatment, or heat treatment.
However, the inventors of the present invention applied the technologies disclosed in the respective JP-A-53-60332, JP-A-2-190483, JP-A-4-88196, and JP-A-3-191093 to hot dip galvanized steel sheets, and found that these technologies cannot improve stably the press-formability. Detail study for the cause of failing in attaining the stable improvement has revealed the following. A hot dip galvanized steel sheet contains Al oxide, and a galvannealed steel sheet contains an irregularly distributed Al oxide and has an increased roughness on the surface of plating layer, thus a desired film cannot be stably formed for both cases even by electrodeposition treatment, dipping treatment, coating oxidation treatment, heat treatment, and the like. Specifically for the galvannealed steel sheet, several micrometers or larger irregular profile on the surface thereof is created owing to the non-uniformity of alloying reaction and to the shape of Fe—Zn alloy phase, thereby increasing the sliding resistance at the surface of plateau to deteriorate the press-formability. Furthermore, the inventors of the present invention determined the friction factor of ZnO film formed on each of the hot dip galvanized steel sheet and the galvannealed steel sheet by a physical method, and found that sufficient press-formability cannot be attained. The findings lead to a conclusion that the conventional technologies of forming a ZnO film on the surface of plating layer cannot expect the sufficient improvement in the press-formability even when a uniform film is formed.
To this point, the inventors of the present invention disclosed a technology to improve the sliding performance in JP-A-2001-323358. According to the disclosure, a plateau is formed on a plating layer on a galvannealed steel sheet, and a film composed of an oxide or a hydroxide containing Zn, Fe, Al, and the like is formed on the plateau, and further a fine irregular profile is formed on the surface of plateau including the film.
Although the technology disclosed in JP-A-2001-323358 improves the press-formability more than the technologies disclosed in the above patent publications, there was occurred insufficient improvement in the press-formability in some cases.
In recent years, the bonding method of hot dip galvanized steel sheets increases the cases of applying adhesives to bonding the steel sheets together. To do this, however, the hot dip galvanized steel sheets have to have strong bonding strength, or have excellent bondability.
The above-described conventional technologies, however, decrease the bondability and the phosphatability, in some cases, by forming a film on the hot dip galvanized steel sheet.